I think if the U-Joint blew apart, it would've beat up the metal. This is just a guess, but back in the late 70's/early 80's, there were aftermarket cruise control units that had magnets that mounted to the driveline with baling wire to create a field for a pickup coil. If it came loose, it would cause that kind of mark as it rotated around with the driveshaft. We won't talk about how I came to this theory just by looking at scratches on the bottom of your car.

Steve

PS. Are those two little screw holes, like you'd mount a bracket with, just to the right of the mark in the picture?

Steve,
I don't remember seeing any holes in that area. Probably just grease specs I missed. The camera flash shows everything!